Google the phrase “Clinton firewall” and you will come up with an ever-lengthening list of scenarios that Hillary Clinton’s presidential campaign has said will stop Barack Obama’s candidacy. The New Hampshire primary, said her campaign, would be the firewall to end Obamamania. Then Super Tuesday was supposed to be the firewall. Then Texas. Now Pennsylvania.

For months, this “just wait until the next states” mantra has diverted our attention from the firewall’s grounding in the taboo subject of black-white racial politics. More specifically, the firewall is anchored in the Race Chasm — a political topography that gets its name from what it looks like on a graph. Here’s how it works, and why it explains Clinton’s tactics.

To date, 42 states and the District of Columbia have voted in primaries or caucuses. Take out the two senators’ home states (Illinois, New York and Arkansas), the two states where Edwards was a major factor (New Hampshire and Iowa) and the one state where only Clinton was on the ballot (Michigan) and that leaves 37 elections where the head-to-head Clinton-Obama matchup has been most clear. Subtract the Latino factor (a hugely important but wholly separate influence on the election) by removing four states whose Hispanic population is over 25 percent (California, New Mexico, Texas and Arizona), and that leaves 33 elections that best represent how the black-white split has impacted the campaign.

When you line up these states left to right from smallest to largest African-American populations, and then chart Obama’s margin of victory or defeat in a those states, a striking pattern emerges (see graphic on Page 6D). That dip in Obama’s performance in states with a big-but-not-huge African-American population is the Race Chasm — and that chasm is no coincidence.

On the graph’s left are states with the smallest black populations, including Western states like Colorado, Idaho, Utah, Washington and Wyoming. Obama has destroyed Clinton in these states, in part because black-white racial politics are essentially non-existent here, and thus Clinton has no inherent advantage. From the now-famous Geraldine Ferraro comment to Bill Clinton likening Obama’s campaign to Jesse Jackson’s, to Clinton aides calling Obama “the black candidate,” the Clinton campaign’s attempts to highlight race have failed in places where such coded messages have not been a significant part of the local political discussion.

On the graph’s right are states with the largest black populations. Obama has crushed Clinton there, too. Unlike the super-white regions, these states — many in the Deep South — have a sordid history of day-to-day, black-white racial politics. “But in the Democratic primary the black vote is so huge [in these states], it can overwhelm the white vote,” says Thomas Schaller, a political science professor at the University of Maryland-Baltimore. That black vote — in part reacting to the Clinton campaign’s tactics — has gone primarily to Obama, helping him win these states by big margins.

It is in the chasm where Clinton has defeated Obama. These are states from Ohio to Oklahoma to Massachusetts, where black-white racial politics are ingrained in the political culture but where the black vote is too small to offset a white vote racially motivated by the Clinton campaign. This chasm exists in states whose population is above 6 percent and below 17 percent black, and Clinton has won them by beating Obama among white working- class voters.

In all, Obama has only eked out victories in three states with Race Chasm demographics — and those are states that provided him unique advantages. He was victorious in Illinois, his home state; Missouri, an Illinois border state; and Connecticut, a state whose Democratic electorate just two years before supported Ned Lamont’s insurgent Democratic primary candidacy against Joe Lieberman, and therefore had well-developed infrastructure to support his outsider candidacy. Meanwhile, three-quarters of all the states Clinton has won are those with Race Chasm demographics.

Pennsylvania Gov. Ed Rendell, a Democrat and a Clinton supporter, publicly noted this dynamic in a February interview when he said Obama’s ethnicity could prevent him from winning the state, which, at 10.6 percent black, falls squarely in the Race Chasm.

“You’ve got conservative whites here, and I think there are some whites who are probably not ready to vote for an African-American candidate,” Rendell said.

Delegate-rich primaries now loom in a critical group of Race Chasm states: Pennsylvania, Indiana (8.8 percent black) and Kentucky (7.5 percent black). Clinton, knowing big wins in the Race Chasm can fortify her firewall, has subsequently intensified her efforts to inject race into the campaign. Most recently, she attacked Jeremiah Wright, Obama’s former pastor who has delivered fiery speeches indicting white racism. She is so determined to raise race issues, in fact, that she gave an in-person interview to the Pittsburgh Tribune-Review specifically to criticize Wright. It was a revealing move, considering the Tribune-Review is the ultra-conservative newspaper owned by the same billionaire who funded the anti-Clinton witchhunts of the ’90s.

Clearly, race is not the only force moving votes. Demographic groups — white, black or any other — do not vote as monoliths. Additionally, the Race Chasm does not mean every white voter who votes against Obama nor every black voter who supports Obama is racially motivated.

That said, with 33 states accounted for, it is impossible to deny the existence of the Race Chasm — particularly considering the regional, cultural and partisan diversity of each state cluster on the graph.

“When the black population is really small, racial polarization is small enough that Obama can win, and when the black population is large, any polarization is drowned out by the overwhelming size of the Democratic black vote,” says Schaller. “But in the middle range, polarization is sizeable enough that black voters cannot overcome it, and these are the states where she wins.”

Under assault by Clinton, Obama has honestly confronted the issue of race. His speech in Philadelphia courageously explored the very gap the Race Chasm illustrates.

Predictably, the Illinois senator’s candor was met by attacks from the same conservative movement that has mastered the use of racism as a political cudgel. Summing up the right’s “don’t-ask-don’t-tell” attitude when it comes to bigotry, New York Times conservative columnist Bill Kristol said, “The last thing we need now is a heated national conversation about race.”

From a cushy office in Washington, D.C. — one of the most segregated cities in America — it must be easy for well-heeled white commentators like Kristol to tell the rest of us that race should be ignored. But as the Race Chasm shows, now is precisely the time we need a national conversation about the divisions that still so clearly afflict our country.

Denver political analyst David Sirota (www.credoaction.com/sirota) is an editor at In These Times magazine, from which this article was adapted. He is author of “The Uprising,” to be published in May, and is a fellow at the Campaign for America’s future. He writes a weekly column for The Post.